The present invention relates to the field of thermal printing. More particularly, it addresses the technical task of eliminating certain artifacts appearing in thermal prints as a result of variations in the tension of a dye-bearing donor web during the printing operation.
In the thermal printing process, a dye-bearing donor web is brought into contact with a dye-receiving print media at a print zone. Thermal printing is effected by contacting the donor web with a multi-element print head which spans the donor web in a direction transverse to the direction of web travel. The print head typically comprises a linear array of closely spaced resistive elements, each being independently addressable by an applied voltage to heat that portion of the donor web directly opposite and thereby cause dye to transfer from the donor web to the print media. To maintain intimate contact between the donor web and print media during this printing operation, the donor web and print media are partially wrapped over the surface of a rotatably-driven platen roller, sometimes referred to as a "transfer drum". The transfer drum is usually driven by a precision stepper motor so that the drum may be stopped at precise locations for printing a line of information on the print media. Most often, the take-up spool is rotatably driven by a far less-expensive DC motor, since its function is simply to accumulate expended donor web. The donor web is supplied by a rotatably mounted supply spool, and a clutching arrangement is used to control the drag on the web by the supply spool so as to prevent free-wheeling of the supply spool under the influence of the take-up spool motor.
In thermal printing apparatus of the above type, it has been observed that the print quality is influenced considerably by the tension in the donor web during printing. When web-tension varies during printing, an artifact known as "banding" appears in the thermal print. This artifact is particularly noticeable when the nominal donor web tension is high, as is ordinarily the case when the take-up spool has not yet accumulated a significant amount of donor web and, hence, the diameter of the wound-up web spool is small. Ideally, the web tension exerted by the take-up spool should be maintained uniform throughout the printing cycle. Unfortunately, however, this ideal is very difficult to achieve, especially when relatively low-cost drive motors are used to effect take-up spool rotation. Web tension is also known to vary with the load applied by the print head, and the drag action of the web supply spool. Also, the relative diameters of the supply and take-up spools have a variable effect on web tension. As prints are made, these spool diameters change, thereby altering the web tension.
In U.S. Pat. No. 4,642,656 issued to Shibaya et al., there is disclosed a thermal printer having a print head which is movable between a printing position in which it contacts the donor web at a print zone, and a non-printing position in which it is spaced from the donor web. According to this disclosure, the torque transmitted to the take-up spool is determined by the position of the thermal print head. When the print head is in its printing position, i.e. contacting the donor web, the transmitted torque to the take-up spool is smaller than that when the print head is in a non-printing position, spaced from the donor web. The apparent intent, here, is to maintain lesser tension on the donor web during printing than during the intervals between prints. While this approach may minimize certain web-tearing problems which may occur when a uniformly high tension is always applied to the web, it does not eliminate the "banding" artifacts which will still appear in the thermal print in the case of web tension variations about a low-tension level.